Generally, a hybrid electric vehicle includes an ETC (Electric Throttle Control) that is applied to a gasoline engine, an electric motor, and a CVT (Continuously Variable Transmission) in order to transmit driving torque to the wheels.
A hybrid electric vehicle can be operated using battery voltage in order to increase fuel economy of the gasoline engine. To generate the battery voltage, kinetic energy of the vehicle is converted to electric energy during a power generation mode, e.g., while braking or decelerating. The battery is charged with the acquired electric energy so that fuel economy can be increased, and to provide power for other vehicle control functions. The battery must be charged to specific voltage levels.
During charging or discharging, the voltage of a battery generally abruptly increases for a short period because of a capacitive component of the battery. Therefore, an instantaneous measurement of the voltage of the battery does not accurately reflect the charge state of the battery while the capacitive component exists. During this time period, exact control of the electric vehicle is difficult.
Capacitive components may not be a problem for low-speed processors for the battery management system, however, because the processor may not detect the voltage of the battery during the existence of the capacitive component. In this case, the battery voltage is detected after the disappearance of the capacitive component. Thus, the acquired voltage (without the capacitive component) reflects the actual charge state of the battery and enables relatively precise control of the battery and the electric vehicle.
However, as the calculation speed of the processor increases, the voltage of the battery is detected even while the capacitive component of the battery exists. Thus, to acquire an accurate effective voltage for a battery, a minimum period of time must pass in order to allow the capacitive component of the battery to disappear. This allows for more stable and more accurate control of the battery and the electric vehicle. In other words, a minimum period of time must pass to allow the abrupt increase of the battery voltage, caused by the capacitive component after charging or discharging, to disappear.
Because the capacitive component of the battery that exists during charging or discharging is not substantial, this voltage cannot be used as a source of power for driving the motor or other voltage-driven function. The capacitive component is not an energy storage component, but rather a component that rapidly responds to a load. The capacitive component acts as a noise component when calculating voltage-dependent values, such as the state of the charge, the current limitation, the available voltage, or the real voltage.
Generally, the capacitive component of the battery can be acquired using the Thevenin equivalent circuit for the battery while the battery is not being charged or discharged. However, modeling of the battery according to this method is not suitable for ascertaining an unloaded voltage, an internal resistance, or a capacitive component. It is much more difficult to estimate the capacitive component of the battery in a hybrid electric vehicle in which charging and discharging occur repeatedly.